The present invention pertains to an inkjet ink set and, more particularly, to a soluble colorant based ink set, comprising at least four differently colored visible inks, wherein none of the visible inks absorb significantly in the near infrared region (IR transparent).
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
There is commercial interest in providing intelligible markings on the surface of articles that are virtually invisible to the human eye, but which can be detected, for example, by a scanning device. Such markings can be used for authentication, sorting and other uses. It is further desirable to combine invisible markings with colored images and in particular colored images created by inkjet printing. The colored images preferably do not interfere with the detection of the invisible markings.
To create invisible markings, it is known to use near-infrared (IR) absorbing and infrared fluorescent compounds which have minimal light absorption in the visible light wavelength range of about 400-700 nm, and which have strong light absorbance in the near infrared wavelength region of about 700-900 nm. These compounds may also have accompanying fluorescence to produce fluorescent radiation having wavelengths longer than the wavelength of excitation. See, for example, U.S. Pat. Nos. 5,093,147, 5,336,174, 5,423,432 and 5,461,136, the disclosures of which are incorporated by reference herein for all purposes as if fully set forth. The infrared absorbance or fluorescence is detected by IR sensitive devices such as cameras and sensors.
Invisible infrared inks have been disclosed for ink jet printing. For example, U.S. Pat. No. 5,990,197 (the disclosure of which is incorporated by reference herein for all purposes as if fully set forth) discloses an organic solvent based polyester ink formulation having an infrared fluorescing compound suitable for ink jet printing.
To achieve full color images, ink jet printers typically employ a cyan, magenta and yellow ink (CMY ink set). These colors are known as subtractively-mixing primaries as light is subtracted as it passes through the colorant. These three colored inks can be used to print the entire range of hues. Inks sets commonly comprise, in addition, a black ink (CMYK ink set).
When used in combination with an IR absorbing/fluorescing marker, an ink set will preferably be non-absorbing (transparent) in the near-infrared region. Many magenta, yellow and black dyes commonly employed in current practice are satisfactorily transparent in this region of the spectrum. The notable exception is the cyan dye.
Cyan colored inkjet inks generally employ a copper phthalocyanine-based chromophore, for example, Direct Blue 199. Phthalocyanines, however, are widely known to absorb in the infrared (see, for example, The Phthalocyanines, Vol 1. Moser et al. CRC Press). At present, there are no practical alternative cyan chromophores that are sufficiently transparent in the near-IR region to be of use in marking applications with IR markers.
There is thus a need in the art, and it is an objective of this invention, to provide an inkjet ink set that can produce a full color gamut and that is substantially transparent in the near-IR region.